2.996/6.971 Biomedical Devices Design Laboratory
Lecture 9: Analog Signal Processing
Important DatesIn-class presentations: Dec. 5 and 10Journal articles due: Dec. 12Presentations at MGH: March 4 and 11 (4-6pm)
Active Filters• Common Filter Tasks
– LPF – Reduce high frequency noise– LPF – Smooth out an envelop– HPF – Block DC signal– BPF – Remove out-of-band noise– Notch filter – Remove interference signal
• Ideal filter characteristics– Flat passband gain– Infinite stop band attenuation
• 1st order filter 20 dB per decade attenuation• What if we want more?
Filter Roll-off Characteristics
• Butterworth– Maximum passband flatness
• Bessel– Constant phase dispersion– Some pass-band attenuation– Slow roll-off
• Tschebyscheff– Maximum roll-off steepness– Passband ripples– Tables available for 1, 2, and
3dB passband ripple
Bessel Butterworth Tschebyscheff (3dB)
R4/R3 0.268 0.568 0.234
Q 0.58 0.71 1.3
2nd Order filter coefficients (Ref: Op Amps for Everyone, R. Mancini, pp 277)
The Sallen-Key LPF(Equal component value form)
( )( ) ( )
02 2
01 3C C
AA sRC A s RC sω ω
=+ − +
1C RC
ω =
40
3
1 RAR
= +
2nd Order HPF and BPF
• Sallen-Key HPF • Multiple Feedback BPF
LTC1563-x Series Pre-packaged LPF• Two internal opamps• Integrated capacitors better accuracy• ~$2 in quantity
Images removed due to copyright restrictions.
LTC1062 – DC Accurate 5th order LPF•Switched cap filter; ~$3 each in quantity
Images removed due to copyright restrictions.
Synchronous Detection• What is it for?
– Measuring small signals in a noisy environment
• How does it work?– Modulate the excitation signal– Demodulate the sensor output– Isolate signal from noise using
the frequency and phase
• When to use it?– Avoiding interference– Sensor noise at DC (1/f noise)– AC excitation
• Remember: DC-coupled sensors are susceptible to drift errors
Synchronous Detection
• Phase shift attenuates the signal by a cosine factor!
Analog Multipliers
• ~$5 each• ±15V Supply• More expensive
versions with better BW and accuracy
All Pass Filter• Unity gain for all frequencies• Phase shift = 180° for• Phase shift = 90° for• Phase shift = 135° at • Switch R and C to get 0-90° phase shift
ω→∞0ω→
1/ RCω =
Microprocessor-based Sync. Det.
• +1/-1 buffers must have low settling time!
Analog-to-Digital Converters (ADC)
• Fundamental tradeoff between resolution and speed (samples per second)
Analog-to-Digital Converters
• Comparators– Differences between a comparator and an Op Amp– Hysteresis
• Architectures– Flash– SAR– Sigma-Delta– Pipelined
• How to use SAR ADC– Flywheel capacitor
• How to use a sigma-delta ADC
Comparators
• A high gain amplifier without feedback• Hysteresis
ADC Topologies
SAR ADCs
• Free-wheel capacitor
Sigma-Delta ADCs
